Segmentation method and method for signaling segmentation of a coding tree unit

ABSTRACT

A method for encoding at least one image, including subdividing the image into a plurality of blocks and subdividing at least one current block into a first portion and a second portion. The first portion has a rectangular or square shape and the second portion complements the first portion in the current block. The second portion has a geometric shape with m sides, wherein m&gt;4. Then the first and second portions are encoded.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is Continuation of U.S. application Ser. No.15/527,548, filed May 17, 2017, which is a Section 371 National StageApplication of International Application No. PCT/FR2015/053196, filedNov. 24, 2015, and published as WO 2016/083729 on Jun. 2, 2016, not inEnglish, the contents of which are incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The present invention relates generally to the field of imageprocessing, and more particularly to the coding and decoding of digitalimages and of sequences of digital images. The invention can be appliedparticularly, but not exclusively, to the video coding implemented inthe current AVC and HEVC video coders, and their extensions (MVC,3D-AVC, MV-HEVC, 3D-HEVC, etc.), as well as to the correspondingdecoding.

BACKGROUND OF THE INVENTION

Current video coders (MPEG, H.264, HEVC, . . . ) use a blockrepresentation of the images to be coded. The images are subdivided intoblocks of square or rectangular form, which can in turn be subdividedrecursively. In the HEVC standard, such a recursive subdivision observesa tree structure called “quadtree”. To this end, as represented in FIG.1, a current image IN is subdivided a first time into a plurality ofsquare or rectangular blocks called CTUs (coding tree units), designatedCTU₁, CTU₂, CTU_(i), . . . , CTU_(L). Such blocks for example have asize of 64×64 pixels (1≤i≤L).

For a given block CTU_(i), it is considered that this block constitutesthe root of a coding tree in which:

-   -   a first level of leaves under the root corresponds to a first        level of depth of subdivision of the block CTU_(i) for which the        block CTU_(i) has been subdivided a first time into a plurality        of square or rectangular coding blocks called CUs (coding        units),    -   a second level of leaves under the first level of leaves        corresponds to a second level of depth of partitioning of the        block CTU_(i) for which some blocks of said plurality of coding        blocks of the block partitioned a first time are partitioned        into a plurality of coding blocks of CU type, . . .    -   . . . a kth level of leaves under the k−1th level of leaves        which corresponds to a kth level of depth of partitioning of the        block CTU_(i) for which some blocks of said plurality of coding        blocks of the block partitioned k−1 times are partitioned one        last time into a plurality of coding blocks of CU type.

In an HEVC-compatible coder, the iteration of the partitioning of theblock CTU_(i) is performed to a predetermined level of depth ofpartitioning.

At the end of the abovementioned successive partitionings of the blockCTU_(i), as represented in FIG. 1, the latter is finally partitionedinto a plurality of coding blocks denoted UC₁, UC₂, . . . , UC_(j), . .. , UC_(M), where 1≤j≤M.

The aim of such a subdivision is to delimit zones which adapt well tothe local characteristics of the image, such as, for example, a uniformtexture, a constant motion, an object in the foreground in the image,etc.

For a block CTU_(i) considered, several different subdivisions of thelatter are placed in competition in the coder, that is to sayrespectively different combinations of subdivision iterations, in orderto select the best subdivision, that is to say the one which optimizesthe coding of the block CTU_(i) considered according to a predeterminedcoding performance criterion, for example the rate/distortion cost orelse an efficiency/complexity compromise, which are criteria well knownto those skilled in the art.

Once a block CTU_(i) considered has been optimally subdivided, asequence of digital item of informations, such as a series of bits forexample, representative of this optimal subdivision, is transmitted in adata signal intended to be stored on the coder or else transmitted to avideo decoder to be read, then decoded thereby.

In the example of FIG. 1, the binary sequence representative of theoptimal subdivision of the block CTU_(i) contains the followingseventeen bits: 1,1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, forwhich:

-   -   the first bit “1” indicates a subdivision of the block CTU_(i)        into four smaller subblocks UC₁, UC₂, UC₃, UC₄,    -   the second bit “1” indicates a subdivision of the subblock UC₁        into four smaller subblocks UC₅, UC₆, UC₇, UC₈,    -   the third bit “0” indicates an absence of subdivision of the        subblock UC₂,    -   the fourth bit “0” indicates an absence of subdivision of the        subblock UC₃,    -   the fifth bit “0” indicates an absence of subdivision of the        subblock UC₄,    -   the sixth bit “0” indicates an absence of subdivision of the        subblock UC₅,    -   the seventh bit “1” indicates a subdivision of the subblock UC₆        into four smaller subblocks UC₉, UC₁₀, UC₁₁, UC₁₂,    -   the eighth bit “1” indicates a subdivision of the subblock UC₇        into four smaller subblocks UC₁₃, UC₁₄, UC₁₆, UC₁₆,    -   the ninth bit “0” indicates an absence of subdivision of the        subblock UC₈,    -   the tenth bit “0” indicates an absence of subdivision of the        subblock UC₉,    -   the eleventh bit “0” indicates an absence of subdivision of the        subblock UC₁₀,    -   the twelfth bit “0” indicates an absence of subdivision of the        subblock UC₁₁,    -   the thirteenth bit “0” indicates an absence of subdivision of        the subblock UC₁₂,    -   the fourteenth bit “0” indicates an absence of subdivision of        the subblock UC₁₃,    -   the fifteenth bit “0” indicates an absence of subdivision of the        subblock UC₁₄,    -   the sixteenth bit “0” indicates an absence of subdivision of the        subblock UC₁₅,    -   the seventeenth bit “0” indicates an absence of subdivision of        the subblock UC₁₆.

The binary sequence obtained requires an order of scanning of thesubblocks to be predetermined in order to know to which subblock asyntax element indicative of the subdivision performed corresponds. Asrepresented by the arrow F in FIG. 1, such an order of scanning isgenerally lexicographic, that is to say that, for each level ofsubdivision considered:

-   -   the subblocks are scanned beginning with the first subblock UC₁        situated top left of the block CTU_(i) and so on until the        subblock UC₄ situated bottom right of the block CTU_(i) is        reached.    -   The subblocks resulting from the subdivision of the subblock UC₆        are scanned beginning with the first subblock UC₉ situated top        left of the subblock UC₆ and so on until the subblock UC₁₂        situated bottom right of the subblock UC₆ is reached,    -   the subblocks resulting from the subdivision of the subblock UC₇        are scanned beginning with the first subblock UC₁₃ situated top        left of the subblock UC₇ and so on until the subblock UC₁₆        situated bottom right of the subblock UC₇ is reached.

The abovementioned seventeen bits are entered one after the other in thebinary sequence which is then compressed by a suitable entropic coding.

For at least one subblock considered out of the various subblocksobtained, a prediction of pixels of the subblock considered isimplemented relative to prediction pixels which belong either to thesame image (intra-prediction), or to one or more preceding images of asequence of images (inter-prediction) which have already been decoded.Such preceding images are conventionally called reference images and areretained in memory both on the coder and on the decoder. During such aprediction, a residual subblock is computed by subtraction, from thepixels of the subblock considered, of the prediction pixels. Thecoefficients of the computed residual subblock are then quantized aftera possible mathematical transformation, for example of discrete cosinetransform (DCT) type, then coded by an entropic coder.

The choice between inter- or intra-prediction mode is made at the levelof each of the subblocks UC₁, UC₂, . . . , UC_(j), . . . , UC_(M) whichcan themselves be partitioned into prediction subblocks (predictionunits) and into transform subblocks (transform units). Each of theprediction subblocks and of the transform subblocks are in turn likelyto be recursively subdivided into subblocks according to theabovementioned “quadtree” tree structure.

The block CTU_(i) and its subblocks UC₁, UC₂, . . . , UC_(j), . . . ,UC_(M), its prediction subblocks and its transform subblocks, are likelyto be associated with information describing their content.

Such information is notably as follows:

-   -   the prediction mode (intra-prediction, inter-prediction, default        prediction producing a prediction for which no information is        transmitted to the decoder (skip));    -   the prediction type (orientation, reference image component,        etc.);    -   the type of subdivision into subblocks;    -   the transform type, for example DCT 4×4, DCT 8×8, etc. . . . ;    -   the pixel values, the transform coefficient values, amplitudes,        signs of quantified coefficients of the pixels contained in the        block or the subblock considered.

This information is also included in the abovementioned data signal.

During the coding of a fixed image or of an image of a sequence ofimages using a subdivision into subblocks of quadtree type, it iscommonplace to retrieve from the image a significant object of averageor small size which is situated in a zone of the image that isrelatively uniform. Such a configuration is for example represented inFIG. 2A which represents, as significant element, a star, which iscontained in a uniform zone such as, for example, a sky of uniformcolor.

After implementation of a subdivision into blocks and into subblocks ofquadtree type as represented in FIG. 2B, it is possible to isolate thesignificant element “star” in a subblock UC₈ suited to its size.

One drawback with such a subdivision is that it requires thetransmission of a binary sequence representative of this subdivisionwhich contains a not-inconsiderable number of bits. Such a sequenceproves costly to signal, which does not make it possible to optimize thereduction of the gain in compression of the coded data. This results inunsatisfactory compression performance levels.

SUBJECT AND SUMMARY OF THE INVENTION

One subject of the present invention relates to a method for coding atleast one image, comprising a step of subdivision of the image into aplurality of blocks.

The coding method according to the invention is noteworthy in that itcomprises the following steps:

-   -   subdividing at least one current block into a first part and a        second part, the first part having a rectangular or square form        and the second part forming the complement of the first part in        the current block, the second part having a geometrical form        with m sides, with m>4,    -   coding the first and second parts.

Such an arrangement makes it possible to very simply subdivide a blockinto only two parts. The binary sequence representative of thissubdivision necessarily contains fewer bits than the binary sequencerepresentative of a subdivision of “quadtree” type. The binary sequencerepresentative of the subdivision according to the invention istherefore much less costly to signal.

Moreover, the subdivision according to the invention is particularlywell suited to the case where blocks of the image contain a significantelement, for example an object in the foreground, which is situated in auniform zone exhibiting a low energy, such as, for example, a backgroundof uniform color, orientation or motion.

Correlatively, the invention relates to a device for coding at least oneimage, comprising a partitioning module for subdividing the image into aplurality of blocks.

Such a coding device is noteworthy in that the partitioning module iscapable of subdividing at least one current block into a first part anda second part, the first part having a rectangular or square form andthe second part forming the complement of the first part in the currentblock, the second part having a geometrical form with m sides, wherem>4,

and in that it comprises a coding module for coding the first and secondparts.

Correspondingly, the invention relates also to a method for decoding adata signal representative of at least one coded image having beensubdivided into a plurality of blocks.

Such a decoding method is noteworthy in that it comprises the followingsteps:

-   -   subdividing at least one current block into a first part and a        second part, the first part having a rectangular or square form        and the second part forming the complement of the first part in        the current block, the second part having a geometrical form        with m sides, where m>4,    -   decoding the first and second parts.

Such an arrangement makes it possible to very simply subdivide a currentblock to be decoded into only two parts, such a subdivision being muchless complex to perform then a subdivision of “quadtree” type.

Moreover, the subdivision according to the invention is particularlywell suited to the case where blocks of the image to be decoded containa significant element, for example an object in the foreground, which issituated in a uniform zone exhibiting a low energy, such as, forexample, a background of uniform color, orientation or motion.

In a particular embodiment, during the step of decoding of the secondpart with m sides of the current block, at least one item of informationof reconstruction of the pixels of the second part with m sides of thecurrent block is set to a predetermined value.

One advantage with such an arrangement lies in the fact that the decoderindependently determines said at least one item of information ofreconstruction of the pixels of the second part with m sides. In otherwords, said at least one corresponding item of information ofreconstruction is advantageously not transmitted in the data signalreceived on the decoder. Thus, the reduction of the signaling cost isoptimized.

According to a variant, said at least one item of information ofreconstruction of the pixels of the second part with m sides of thecurrent block is representative of the absence of subdivision of thesecond part with m sides of the current block.

Advantageously, at the moment of decoding the second part with m sidesof the current block, the decoder independently determines that it doesnot need to subdivide this part, since it characterizes a uniform regionof the current block to be decoded which is without detail.

According to another variant, said at least one item of information ofreconstruction of the pixels of the second part with m sides of thecurrent block is representative of the absence of residual informationresulting from a prediction of the pixels of the second part with msides of the current block.

Advantageously, at the moment of decoding the second part with m sidesof the current block, the decoder independently determines that theresidual pixels obtained following the prediction of said second partwith m sides have a zero value. It is considered that the second partwith m sides is associated with a zero prediction residue since itcharacterizes a uniform region of the current block to be decoded.

According to yet another variant, said at least one item of informationof reconstruction of the pixels of the second part with m sides of thecurrent block is representative of predetermined prediction values ofthe pixels of the second part with m sides of the current block.

Such a variant makes it possible to even further optimize the signalingcost by avoiding transmitting, in the data signal, the index of theprediction mode which was selected in the coding to predict the secondpart with m sides of the current block.

In another particular embodiment, the decoding method comprises, priorto the step of subdivision of the current block, a step of reading, inthe data signal, an item of information indicating whether the currentblock is intended either to be subdivided into a first part and a secondpart, the first part having a rectangular or square form and the secondpart forming the complement of the first part in the current block, thesecond part having a a geometrical form with m sides, where m>4, or tobe subdivided according to another predetermined method.

Such an arrangement enables the decoder to determine whether, during thecoding of a current block, the coder activated or did not activate thesubdivision of the current block in accordance with the invention, for asequence of images considered, for an image considered or even for animage portion (slice) considered, such that the decoder cancorrespondingly implement the subdivision performed in the coding. Theresult thereof is that such a decoding method is particularly flexible,because it can be adapted to the current video context. In effect, thedecoding method is adapted to implement the subdivision according to theinvention or according to another type of subdivision, such as, forexample, the quadtree subdivision, according to the value taken by adedicated indicator included in the data signal.

Such a dedicated indicator is still relatively compact to signal andmakes it possible to maintain the compression gain obtained by virtue ofthe subdivision according to the invention.

In yet another particular embodiment, the decoding method comprises astep of reading, in the data signal, an item of information indicating asubdivision configuration of the current block selected from differentpredetermined subdivision configurations.

Such an arrangement makes it possible to adapt the subdivision accordingto the invention according to the location of the significant element inthe uniform region of the current block.

In yet another particular embodiment, the step of decoding of the secondpart with m sides of the current block comprises the substeps consistingin:

-   -   applying an entropic decoding to the pixels of the second part        with m sides,    -   complementing the entropically decoded pixels of the second part        with m sides, with pixels reconstructed according to a        predetermined reconstruction method, until a square or        rectangular block of pixels is obtained.

Such an arrangement advantageously makes it possible, when a step ofapplication of a transform has to be implemented following the step ofentropic decoding of the second part with m sides of the current blockto be decoded, to re-use the hardware and software square or rectangularblock transform tools which are routinely implemented in the currentvideo coders and decoders.

In yet another particular embodiment, a subdivided current blockcontains at most a part having a geometrical form with m sides.

Such an arrangement is well suited to the case where the current blockcontains two zones of quite distinct texture, that is to say the onedefined by a single significant element and the one defined by a singleuniform zone. Advantageously, it is not therefore necessary to proceedwith a new subdivision of the current block to be decoded.

The abovementioned various embodiments or features can be addedindependently or in combination with one another, to the steps of thedecoding method as defined above.

Correlatively, the invention relates to a device for decoding a datasignal representative of at least one coded image having been subdividedinto a plurality of blocks.

Such a decoding device is noteworthy in that it comprises:

-   -   a partitioning module for subdividing at least one current block        into a first part and a second part, the first part having a        rectangular or square form and the second part forming the        complement of the first part in the current block, the second        part having a geometrical form with m sides, where m>4,    -   a decoding module for decoding the first and second parts.

The invention also relates to a computer program comprising instructionsfor implementing one of the coding and decoding methods according to theinvention, when it is run on a computer.

Such a program can use any programming language, and be in the form ofsource code, object code, or intermediate code between source code andobject code, such as in a partially compiled form, or in any otherdesirable form.

Yet another subject of the invention also targets a computer-readablestorage medium, and comprising computer program instructions asmentioned above.

The storage medium can be any entity or device capable of storing theprogram. For example, the medium can comprise a storage means, such as aROM, for example a CD ROM or a microelectronic circuit ROM, or even amagnetic storage means, for example a USB key or a hard disk.

Also, such a storage medium can be a transmissible medium such as anelectrical or optical signal, which can be routed via an electrical oroptical cable, wirelessly or by other means. The program according tothe invention can in particular be downloaded over a network of Internettype.

Alternatively, such a storage medium can be an integrated circuit inwhich the program is incorporated, the circuit being adapted to executethe method concerned or to be used in the execution thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages will become apparent on reading severalpreferred embodiments described with reference to the figures in which:

FIG. 1 represents an example of conventional subdivision of a currentblock, such as the subdivision of “quadtree” type,

FIGS. 2A and 2B represent an application of the subdivision of“quadtree” type to a current block which contains a single significantelement, a star, which is contained in a uniform zone such as, forexample, a sky of uniform color,

FIG. 3 represents the main steps of the coding method according to anembodiment of the invention,

FIG. 4 represents an embodiment of a coding device according to theinvention,

FIGS. 5A to 5J respectively represent different embodiments ofsubdivision according to the invention of the current block,

FIGS. 6A and 6B respectively represent two embodiments of coding of theparts obtained by subdivision of the current block, in accordance with atype of subdivision represented in FIG. 5A,

FIG. 7 represents an example of subdivision of the current block towhich the coding embodiment of FIG. 6B is applied,

FIG. 8 represents the main steps of the decoding method according to anembodiment of the invention,

FIG. 9 represents an embodiment of a decoding device according to theinvention,

FIGS. 10A and 10B respectively represent two embodiments of decoding ofthe parts obtained after reconstruction of the subdivision of thecurrent block, in accordance with a type of subdivision represented inFIG. 5A.

DETAILED DESCRIPTION OF THE CODING PART

An embodiment of the invention will now be described, in which thecoding method according to the invention is used to code an image or asequence of images according to a binary signal similar to that which isobtained by a coding implemented in a coder conforming to any one of thecurrent or future video coding standards.

In this embodiment, the coding method according to the invention is forexample implemented by software or hardware by modifications to such acoder. The coding method according to the invention is represented inthe form of an algorithm comprising steps C1 to C7 as represented inFIG. 3.

According to the embodiment of the invention, the coding methodaccording to the invention is implemented in a coding device or coder COrepresented in FIG. 4.

As illustrated in FIG. 4, such a coder comprises a memory MEM_COcomprising a buffer memory TAMP_CO, a processing unit UT_CO equipped forexample with a microprocessor μP and driven by a computer program PG_COwhich implements the coding method according to the invention. Oninitialization, the code instructions of the computer program PG_CO arefor example loaded into a RAM memory (not represented) before beingexecuted by the processor of the processing unit UT_CO.

The coding method represented in FIG. 3 is applied to any current imageIC_(j) fixed or indeed forming part of a sequence of L images IC₁, . . ., IC_(j), . . . , IC_(L) (1≤j≤L) to be coded.

During a step C1 represented in FIG. 3, as is known per se, a currentimage IC_(j) is subdivided into a plurality of blocks of abovementionedCTU type: CTU₁, CTU₂, . . . , CTU_(u), . . . , CTU_(S) (1≤u≤S).

Such a step of subdivision is implemented by a processor or partitioningsoftware module MP_CO represented in FIG. 4, which module is driven bythe microprocessor μP of the processing unit UT_CO.

Preferentially, each of the blocks CTU₁, CTU₂, . . . , CTU_(u), . . . ,CTU_(S) has a square form and comprises N×N pixels, where N≥2.

According to an alternative, each of the blocks CTU₁, CTU₂, . . . ,CTU_(u), . . . , CTUs has a rectangular form and comprises N×P pixels,where N≥1 and P≥2.

During a step C2 represented in FIG. 3, for a previously selectedcurrent block CTU the partitioning module MP_CO of FIG. 4 subdivides thecurrent block CTU into at least one first part and one second part, thefirst and second parts complementing one another. According to theinvention:

-   -   the first part has a rectangular or square form,    -   and the second part has a geometrical form with m sides, where        m>4.

According to a preferred embodiment, the current block CTU issubdivided:

-   -   into a first part of rectangular or square form or else into a        plurality of parts of rectangular or square form,    -   and into at most one second part of a geometrical form with m        sides.

In the sense of the invention, the first and second parts respectivelyform two distinct coding units CU₁ and CU₂. The latter terminology isnotably used in the HEVC standard “ISO/IEC/23008-2 ITU-T RecommendationH.265, High Efficiency Video Coding (HEVC)”.

According to a first embodiment of subdivision represented in FIG. 5A,for a square current block CT_(u) of size N×N:

-   -   the first part CU₁ is a square block of size N/2×N/2,    -   and the second part CU₂, which forms the complement of the first        part CU₁ in the current block CTU_(u), has a geometrical form        with m sides, where m>4.

In the example represented in FIG. 5A, m=6.

As represented in FIG. 5A, four types of subdivision SUB1 ₁, SUB2 ₁,SUB3 ₁, SUB4 ₁ of the current block CT_(u) are possible, the squareblock CU₁ being able to be situated in one of the four corners of thecurrent block CTU_(u).

In the interests of clarity of FIG. 5A, only in the case for example ofthe types of subdivision SUB1 ₁ and SUB3 ₁, there are represented:

-   -   the first pixel ptl_(u) of the current block CTU_(u), of        coordinates (x_(min), y_(min)), which is situated top left        therein,    -   the last pixel pbr_(u) of the current block CTU_(u), of        coordinates (x_(max), y_(max)), which is situated bottom right        therein,    -   the first pixel ptl₁ of the first part CU₁, of coordinates        (x′_(min), y′_(min)), which is situated top left therein,    -   the last pixel pbr₁ of the first part CU₁, of coordinates        (x′_(max), y′_(max)), which is situated bottom right therein.

According to the particular type of subdivision SUB1 ₁, the first pixelptl_(u) of the current block CTU is the same as the first pixel ptl₁ ofthe first part CU₁.

Whatever the type of subdivision chosen, the second part CU₂ of ageometrical form with m sides is then defined generally as a set ofpixels ptl₂ such that, for any pixel pv₂(x″_(v), y″_(v)) of this set:

-   -   x_(min)≤x″_(v)≤x_(max) and y_(min)≤y″_(v)≤y_(max)    -   x″_(v)<x′_(min) or x″_(v)>x′_(max) or y″_(v)<y′_(min) or        y″_(v)>y′_(max)

According to a second embodiment of subdivision represented in FIG. 5B,for a square current block CTU of size N×N:

-   -   the first part CU₁ is a square block of size

${\frac{N}{4} \times \frac{N}{4}},$

-   -   and the second part CU₂, which forms the complement of the first        part CU₁ in the current block CTU_(u), has a geometrical form        with m sides, where m>4.

In the example represented in FIG. 5B, m=6 or m=8.

As represented in FIG. 5B, sixteen types of subdivision SUB1 ₂, SUB2 ₂,. . . , SUB16 ₂ of the current block CTU_(u) are possible, the squareblock CU₁ being able to be situated in sixteen different positionswithin the current block CTU_(u), by successive translation of N/4pixels of the square block CU₁ within the current block CTU_(u).

In the interests of clarity of FIG. 5B, only in the case, for example,of the types of subdivision SUB1 ₂ and SUB9 ₂, there are represented:

-   -   the first pixel ptl_(u) of the current block CTU_(u), of        coordinates (x_(min), y_(min)), which is situated top left        therein,    -   the last pixel pbr_(u) of the current block CTU_(u), of        coordinates (x_(max), y_(max)), which is situated bottom right        therein,    -   the first pixel ptl₁ of the first part CU₁, of coordinates        (x′_(min), y′_(min)), which is situated top left therein,    -   the last pixel pbr₁ of the first part CU₁, of coordinates        (x′_(max), y′_(max)), which is situated bottom right therein.

According to the particular mode of subdivision SUB1 ₂, the first pixelptl_(u) of the current block CTU_(u) is the same as the first pixel ptl₁of the first part CU₁.

Whatever the type of subdivision chosen, the second part CU₂ of ageometrical form with m sides is then defined generally as a set ofpixels such that, for any pixel pv₂(x″_(v), y″_(v)) of this set:

-   -   x_(min)≤x″_(v)≤x_(max) and y_(min)≤y″_(v)≤y_(max)    -   x″_(v)<x′_(min) or x″_(v)>x′_(max) or y″_(v)<y′_(min) or        y″_(v)>y′_(max)

According to third, fourth, fifth and sixth embodiments of subdivisionrepresented respectively in FIGS. 5C, 5D, 5E and 5F, for a squarecurrent block CTU of size N×N:

-   -   the first part CU₁ is a rectangular block of size U×V pixels,        such that U<N and V<N, the set of the coordinates of such a        rectangular block being chosen from a predefined list LT_(a) of        several sets of coordinates each defining a rectangular block of        a predetermined form, the list LT_(a) being stored in the buffer        memory TAMP_CO of the coder CO of FIG. 4,    -   and the second part CU₂, which forms the complement of the first        part CU₁ in the current block CTU_(u), has a geometrical form        with m sides, where m>4.

In each of the FIGS. 5C to 5F, a single type of subdivision of thecurrent block CTU_(u) has been represented, bearing in mind that therecan obviously be several thereof.

Furthermore, the definition of the second part CU₂ of the current blockCTU is the same as that given in the examples of FIGS. 5A and 5B.

According to seventh, eighth, ninth and tenth embodiments of subdivisionrepresented respectively in FIGS. 5H, 5I, 5J and 5K, the current blockCTU_(u) is a rectangle of size N×P pixels, where N≥1 and P≥2.

According to these four subdivision modes:

-   -   the first part CU₁ is a rectangular block of size U×V, such that        U<N and V<P, the set of the coordinates of such a rectangular        block being chosen from a predefined list LT_(b) of several sets        of coordinates each defining a rectangular block of a        predetermined form, the list LT_(b) being stored in the buffer        memory TAMP_CO of the coder CO of FIG. 4,    -   and the second part CU₂, which forms the complement of the first        part CU₁ in the current block CTU_(u), has a geometrical form        with m sides, where m>4.

In each of the FIGS. 5G to 5I, a single type of subdivision of thecurrent block CTU has been represented, bearing in mind that there canobviously be several thereof.

Furthermore, the definition of the second part CU₂ of the current blockCTU is the same as that given in the examples of FIGS. 5A and 5B.

During a step C3 represented in FIG. 3, each of the current blocksCTU_(u), or only a part, which has been subdivided in accordance withthe different subdivision modes according to the invention asrepresented in FIGS. 5A to 5K is placed in competition:

-   -   with different current blocks CTU_(u) subdivided respectively        according to different well known subdivision modes, such as,        for example, subdivided into only four rectangular or square        blocks, subdivided according to the “quadtree” method, etc.,    -   and with a non-subdivided current block CTU_(u).

Such competition is implemented according to a coding performancecriterion predetermined for the current block CTU_(u), for example therate/distortion cost or else an efficiency/complexity compromise, whichare criteria well known to those skilled in the art.

The competition is implemented by a processor or computation softwaremodule CAL1_CO represented in FIG. 4, which module is driven by themicroprocessor μP of the processing unit UT_CO.

At the end of the competition, an optimal subdivision mode SUB_(opt) ofthe current block CTU_(u) is selected, that is to say that it is the onewhich optimizes the coding of the block CTU_(u) by minimization of therate/distortion cost or else by maximization of theefficiency/complexity compromise.

During a step C4 represented in FIG. 3, an indicator representative ofthe subdivision mode selected on completion of the step C3 is selectedfrom a look-up table TC stored in the buffer memory TAMP_CO of the coderCO of FIG. 4.

Such a selection is implemented by a processor or computation softwaremodule CAL2_CO represented in FIG. 4, which module is driven by themicroprocessor μP of the processing unit UT_CO.

The indicator representative of a given subdivision mode is for examplea syntax element called type_decoupe which, according to a preferentialembodiment, for example takes three values:

-   -   0 to indicate a conventional subdivision of the current block        into four rectangular or square blocks,    -   1 to indicate a subdivision of the current block in accordance        with the subdivision mode represented in FIG. 5A,    -   2 to indicate a subdivision of the current block in accordance        with the subdivision mode represented in FIG. 5B,    -   3 to indicate an absence of subdivision of the current block.

Moreover, in the case where the syntax element type_decoupe has thevalue 1, the latter is associated, in the lookup table TC of FIG. 4,with another syntax element called arr_decoupe1 which indicates the typeof subdivision SUB1 ₁, SUB2 ₁, SUB3 ₁, SUB4 ₁ chosen, as represented inFIG. 5A. The syntax element arr_decoupe1 takes the value:

-   -   0 to indicate the subdivision type SUB1 ₁,    -   1 to indicate the subdivision type SUB2 ₁,    -   2 to indicate the subdivision type SUB3 ₁,    -   3 to indicate the subdivision type SUB4 ₁.

Moreover, in the case where the syntax element type_decoupe has thevalue 2, the latter is associated, in the lookup table TC of FIG. 4,with another syntax element called arr_decoupe2 which indicates the typeof subdivision chosen from the sixteen types of subdivision SUB1 ₂, SUB2₂, . . . , SUB16 ₂ of the current block CTU_(u), as represented in FIG.5B. The syntax element arr_decoupe2 takes the value:

-   -   0 to indicate the subdivision type SUB1 ₂,    -   1 to indicate the subdivision type SUB2 ₂,    -   2 to indicate the subdivision type SUB3 ₂,    -   3 to indicate the subdivision type SUB4 ₂,    -   4 to indicate the subdivision type SUB5 ₂,    -   5 to indicate the subdivision type SUB6 ₂,    -   6 to indicate the subdivision type SUB7 ₂,    -   7 to indicate the subdivision type SUB8 ₂,    -   8 to indicate the subdivision type SUB9 ₂,    -   9 to indicate the subdivision type SUB10 ₂,    -   10 to indicate the subdivision type SUB11 ₂,    -   11 to indicate the subdivision type SUB12 ₂,    -   12 to indicate the subdivision type SUB13 ₂,    -   13 to indicate the subdivision type SUB14 ₂,    -   14 to indicate the subdivision type SUB15 ₂,    -   15 to indicate the subdivision type SUB16 ₂.

During a step C5 represented in FIG. 3, the value of the syntax elementtype_decoupe which was selected on completion of the abovementioned stepC4 is coded, together, if appropriate, with the coding of the syntaxelement arr_decoupe1 or arr_decoupe2 which is associated with it.

The abovementioned step C5 is implemented by a processor or indicatorcoding software module MCI such as represented in FIG. 4, which moduleis driven by the microprocessor μP of the processing unit UT_CO.

During a step C6 represented in FIG. 3, the parts CU₁ and CU₂ of thecurrent block CTU_(u) are coded in a predetermined scan order. Accordingto a preferred embodiment, the first part CU₁ is coded before the secondpart CU₂. Alternatively, the first part CU₁ is coded after the secondpart CU₂.

The coding step C6 is implemented by a processor or coding softwaremodule UCO as represented in FIG. 4, which module is driven by themicroprocessor μP of the processing unit UT_CO.

As represented in more detail in FIG. 4, the coding module UCOconventionally comprises:

-   -   a prediction processor or software module PRED_CO,    -   a residual data computation processor or software module        CAL3_CO,    -   a transformation processor or software module MT_CO of DCT        (discrete cosine transform), DST (discrete sine transform), DWT        (discrete wavelet transform) type    -   a quantization processor or software module MQ_CO,    -   an entropic coding processor or software module MCE_CO, for        example of CABAC (context adaptive binary arithmetic coder”)        type or even a Huffman coder known as such.

During a step C7 represented in FIG. 3, a data signal F is constructedwhich contains the data coded on completion of the abovementioned stepsC5 and C6. The data signal F is then transmitted by a communicationnetwork (not represented) to a remote terminal. The latter comprises adecoder which will be described later in the description.

The step C7 is implemented by a data signal construction processor orsoftware module MCF, as represented in FIG. 4.

The coding steps which have just been described above are implementedfor all the blocks CTU₁, CTU₂, . . . CTU_(u), . . . , CTU_(S) to becoded of the current image IC_(j) considered, in a predetermined orderwhich is, for example, the lexicographic order.

Other types of scanning than that which has just been described aboveare of course possible.

There now follows a description, referring to FIG. 6A, of a firstembodiment of the different substeps implemented during theabovementioned coding step C6, in the coding module UCO represented inFIG. 4.

According to this first embodiment, the optimal subdivision modeSUB_(opt) selected on completion of the coding step C3 is for exampleone of the subdivision modes represented in FIG. 5A. To this end, it isthe indicator type_decoupe of value 1 which was selected on completionof the abovementioned step C4. More specifically, it is for example thesubdivision type SUBD2 ₁, as represented in FIG. 5A, which was selectedon completion of the coding step C3. To this end, the indicatortype_decoupe of value 1 is also associated with the indicatorarr_decoupe1 of value 1, as defined above in the description.

The value 1 of the indicator type_decoupe is entered in compressed forminto the data signal F, followed by the value 1 of the indicatorarr_decoupe1.

Moreover, according to the first embodiment of FIG. 6A, the parts CU₁and CU₂ of the current block CTU are not subdivided again.

To this end, according to one embodiment:

-   -   the indicator type_decoupe of value 3 is associated with the        coded data of the first part CU₁,    -   the indicator type_decoupe of value 3 is associated with the        coded data of the second part CU₂.

According to the invention, the value of the indicator type_decoupeassociated with the coded data of the second part CU₂ is entered incompressed form into the data signal F before the value of the indicatortype_decoupe associated with the coded data of the first part CU₁.

In the example of FIG. 6A, the data signal F therefore contains thefollowing values: 1133 which are representative of the partitioning ofthe current block CTU_(u). s a variant, given the fact that the secondpart CU₂ defines a uniform zone of the current block CTU_(u), noindicator representative of the absence of subdivision of the part CU₂is entered into the data signal F represented in FIGS. 3 and 4.According to such a variant, it is in fact assumed in the coding, as inthe decoding, that an m-sided part of the current block is notsystematically subdivided. Thus, the transmission to the decoder of anindicator type_decoupe of value 3 does not prove necessary.

The data signal F therefore contains the following values: 113, whichreduces the signaling cost.

During a substep C610 represented in FIG. 6A, the coding module UCOselects, as current part CU_(k) (k=1 or k=2), either the square part CU₁first, or the m-sided part CU₂ first.

During a substep C611 represented in FIG. 6A, the PRED_CO module of FIG.4 proceeds with the predictive coding of the current part CU₁.

Conventionally, the pixels of the part CU₁ are predicted relative to thepixels that have already been coded then decoded, by having known intra-and/or inter-prediction techniques compete.

Among the possible predictions for the current part CU₁, the optimalprediction is chosen according to a rate-distortion criterion well knownto those skilled in the art.

Said abovementioned predictive coding substep makes it possible toconstruct a predicted part CUp₁ which is an approximation of the currentpart CU₁. The information relating to this predictive coding willsubsequently be entered into the data signal F represented in FIGS. 3and 4. Such information notably comprises the prediction type (inter- orintra-prediction), and, if appropriate, the intra-prediction mode orelse the reference image index and the motion vector used in theinter-prediction mode. Such information is compressed by the coder COrepresented in FIG. 3.

During a substep C612, the computation module CAL3_CO of FIG. 4 proceedsto subtract the predicted part CUp₁ from the current part CU₁ to producea residual part CUr₁.

During a substep C613 represented in FIG. 6A, the module MT_CO of FIG. 4proceeds to transform the residual part CUr₁ according to a conventionaldirect transformation operation, such as, for example, a discrete cosinetransformation of DCT type, to produce a transform part CUt₁.

During a substep C614 represented in FIG. 6A, the module MQ_CO of FIG. 4proceeds to quantize the transform part CUt₁ according to a conventionalquantization operation, such as, for example, a scalar quantization. Apart CUq₁, formed by quantized coefficients, is then obtained.

During a substep C615 represented in FIG. 6A, the module MCE_CO of FIG.4 proceeds with the entropic coding of the quantized coefficients CUq₁.

The abovementioned substeps C611 to C615 are then iterated in order tocode the m-sided second part CU₂ of the current block CTU_(u).

According to the invention, in the case of the coding of the m-sidedsecond part CU₂, one or more items of information on coding of thepixels of the second part CU₂ are set to predetermined values.

Thus, according to a preferred variant embodiment, during the substepC611 of predictive coding of the part CU₂ of the current block CTU_(u),the pixels of the part CU₂ are predicted relative, respectively, topixels of predetermined corresponding values. Such values are stored ina list LP contained in the buffer memory TAMP_CO of the coder CO of FIG.4.

Preferably, these predetermined prediction values are selected in such away that, during the substep C612 of FIG. 6A, the subtraction of thepredicted part CUp₂ from the current part CU₂ produces a residual partCUr₂ which comprises pixel values that are zero or close to zero.

Such an arrangement makes it possible to advantageously exploit theuniformity of the part CU₂ of the current block CTU_(u) while making itpossible to substantially reduce the signaling cost of the codinginformation of the current block CTU_(u) in the data signal F.

As a variant, the pixels of the part CU₂ are predicted conventionally,in the same way as the part CU₁.

According to another preferred variant embodiment, the quantizedcoefficients of the quantized residual part CUq₂ obtained on completionof the substep C614 of FIG. 6A are all set to zero and are not enteredinto the data signal F.

Such an arrangement makes it possible to advantageously exploit theuniformity of part CU₂ of the current block CTU while making it possibleto substantially reduce the signaling cost of the coding information ofthe current block CTU in the data signal F.

According to the invention, between the abovementioned substeps C612 andC613, an intermediate substep C6120 is implemented. During thisintermediate substep, the residual pixels of the m-sided residual partCUr₂ are complemented with pixels of predetermined respective value,until a square or rectangular block of pixels is obtained.

According to different possible embodiments, the residual pixels of theresidual part CUr₂ can be complemented:

-   -   with pixels of zero respective value,    -   with pixels reconstructed conventionally by interpolation,    -   with pixels reconstructed conventionally using the so-called        “inpaiting” technique.

The abovementioned substep C6120 is implemented by a computationprocessor or software module CAL4_CO as represented in FIG. 4, whichmodule is driven by the microprocessor μP of the processing unit UT_CO.

Such an arrangement makes it possible to re-use the transformationmodule MT_CO of FIG. 4 which conventionally applies square orrectangular block transforms.

Given the fact that the substep C612 is applied only for the second partCU₂ of a geometrical form with m sides, this substep, and thecomputation module CAL4_CO, are represented by dotted lines,respectively in FIGS. 3 and 4.

There now follows a description, referring to FIG. 6B, of a secondembodiment of the different substeps implemented during theabovementioned coding step C6, in the coding module UCO represented inFIG. 4.

This second embodiment is distinguished from that of FIG. 6A by the factthat the first part CU₁ of the current block CTU_(u) is subdividedagain. An example of such a subdivision of the current block CTU_(u) isrepresented in FIG. 7.

In the example of FIG. 7, the optimal subdivision mode SUB_(opt) whichwas selected on completion of the abovementioned coding step C3 is, forexample, once again the indicator type_decoupe of value 1 which wasselected on completion of the abovementioned step C4. As represented inFIG. 7, this value is entered in compressed form into the data signal F.As explained above, the indicator type_decoupe of value 1 is alsoassociated with the indicator arr_decoupe1 of value 1, as defined abovein the description. As represented in FIG. 7, the value of the indicatorarr_decoupe1 of value 1 is then entered in compressed form into the datasignal F following the value of the indicator type_decoupe.

According to the second embodiment of FIG. 6B, in the same way as in theembodiment of FIG. 6A, the second part CU₂ of the current block CTU_(u)is not subdivided again by starting from the principle that it isrepresentative of a uniform zone of the current block CTU_(u).

Together with the coded data of the second part CU₂, the value 3 of theindicator type_decoupe is entered in compressed form into the datasignal F, following the value 1 of the indicator arr_decoupe1. Thisvalue is represented in bold in FIG. 7.

According to the invention, the value of the indicator type_decoupeassociated with the coded data of the second part CU₂ is entered incompressed form into the data signal F systematically before the valueof the indicator type_decoupe associated with the coded data of thefirst part CU₁.

As a variant, the value of the indicator type_decoupe associated withthe coded data of the second part CU₂ could be entered in compressedform into the data signal F systematically after the value of theindicator type_decoupe associated with the coded data of the first partCU₁.

In the example of FIG. 7, the part CU₁ is subdivided, for example intofour square blocks CU1 ₁, CU2 ₁, CU3 ₁, CU4 ₁, according to aconventional subdivision method, of “quadtree” type for example.

The coded data of the part CU₁ are therefore also associated with theindicator type_decoupe of value 0, representative of such a subdivision,as defined above in the description. As represented in FIG. 7, thisvalue is entered in compressed form into the data signal F, followingthe value 3 of the indicator type_decoupe.

In the example of FIG. 7, the block CU1 ₁ is not subdivided. The codeddata of the part CU₁ are therefore also associated with the indicatortype_decoupe of value 3, representative of the absence of such asubdivision, as defined above in the description. As represented in FIG.7, this value is entered in compressed form into the data signal F,following the value 0 of the indicator type_decoupe.

In the example of FIG. 7, the block CU2 ₁ is subdivided according to theinvention, notably according to the type of subdivision SUB6 ₂represented in FIG. 5B. Thus, the block CU2 ₁ is subdivided into a firstpart CU21 ₁ of square form and into an m-sided second part CU22 ₁. Inthe example represented, the second part CU22 ₁ has 8 sides.

The coded data of the part CU₁ are therefore also associated with theindicator type_decoupe of value 2, which is itself associated with theindicator arr_decoupe2 of value 6, as defined above in the description.As represented in FIG. 7, these values 2 and 6 are entered successivelyin compressed form into the data signal F, following the value 3 of theindicator type_decoupe.

In the example of FIG. 7, the block CU3 ₁ is subdivided into four squareblocks CU31 ₁, CU32 ₁, CU33 ₁, CU34 ₁, according to a conventionalsubdivision method, of “quadtree” type for example.

The coded data of the part CU₁ are therefore associated also with theindicator type_decoupe of value 0, representative of such a subdivision,as defined above in the description. As represented in FIG. 7, thisvalue is entered in compressed form into the data signal F, followingthe value 6 of the indicator arr_decoupe2.

In the example of FIG. 7, the block CU4 ₁ is not subdivided.

The coded data of the part CU₁ are therefore associated also with theindicator type_decoupe of value 3, representative of the absence of sucha subdivision, as defined above in the description. As represented inFIG. 7, this value is entered in compressed form into the data signal F,following the value 0 of the indicator type_decoupe.

The second part CU22 ₁ of the block CU2 ₁ is not subdivided again,starting from the principle that it is representative of a uniform zoneof this block.

Together with the coded data of the first part CU₁, the value 3 of theindicator type_decoupe is then entered in compressed form into the datasignal F, following the value 3 of the indicator type_decoupe. Thisvalue is represented in bold in FIG. 7.

According to the invention, the value of the indicator type_decoupeassociated with the m-sided part CU22 ₁ of the block CU2 ₁ is entered incompressed form into the data signal F systematically before the valueof the indicator type_decoupe associated with the square part CU21 ₁ ofthe block CU2 ₁.

As a variant, the value of the indicator type_decoupe associated withthe m-sided part CU22 ₁ of the block CU2 ₁ could be entered incompressed form into the data signal F systematically after the value ofthe indicator type_decoupe associated with the square part C21 ₁ of theblock CU2 ₁.

In the example of FIG. 7, the first part CU21 ₁ of the block CU2 ₁ isnot subdivided. Together with the coded data of the first part CU₁, thevalue 3 of the indicator type_decoupe is then entered in compressed forminto the data signal F, following the value 3 of the indicatortype_decoupe associated with the m-sided part CU22 ₁ of the block CU2 ₁.

In the example of FIG. 7, the four blocks CU31 ₁, CU32 ₁, CU33 ₁, CU34 ₁of the block CU3 ₁ are not subdivided. The value 3 of the indicatortype_decoupe is then entered in compressed form successively four timesinto the data signal F, following the value 3 of the indicatortype_decoupe associated with the part CU21 ₁ of the block CU2 ₁.

As a variant to this second embodiment, the two values 3 of theindicator type_decoupe as represented in bold in FIG. 7 andrepresentative of the absence of subdivision of the m-sided parts CU₂and CU22 ₁ of the current block CTU are not entered into the data signalF, which makes it possible to reduce the signaling cost. It is in factassumed, in the coding as in the decoding, that an m-sided part of thecurrent block is not systematically subdivided. Thus, the transmissionto the decoder of an indicator type_decoupe of value 3 does not provenecessary.

Reference is once again made to FIG. 6B.

During a substep C620 represented in FIG. 6B, the coding module UCOselects, as current part CU_(k) (k=1 or k=2), either the square part CU₁first, or the m-sided part CU₂ first.

During a substep C621 represented in FIG. 6B, the coding module UCOtests whether the index k associated with the current part CU_(k) hasthe value 1 or 2.

If the index k is equal to 2, the part CU₂ of the current block CTU iscoded according to the substeps C611 to C615 of FIG. 6A.

If the index k is equal to 1, during a substep C622 represented in FIG.6B, the coding module UCO of FIG. 4 selects a current subpart CU_(k′) ofthe first part CU₁ of the current block CTU_(u), such that 1≤k′≤N.

In the example represented in FIG. 7, N=8, since the first part CU₁ ofthe current block CTU has been subdivided into eight subparts of “codingunit” type CU1 ₁, CU21 ₁, CU22 ₁, CU31 ₁, CU32 ₁, CU33 ₁, CU34 ₁, CU4 ₁.

During a substep C623 represented in FIG. 6B, the PRED_CO module of FIG.4 selects, for this current subpart CU_(k′) an inter- orintra-prediction mode, for example by having these modes competeaccording to a rate-distortion criterion. The prediction mode selectedis associated with an indicator IPR which is intended to be transmittedin the data signal F.

During an optional substep C624 represented in FIG. 6B, the partitioningmodule MP_CO of FIG. 4 subdivides the current subpart CU_(k′) into aplurality W of prediction subparts PU₁, PU₂, . . . , PU_(z), . . .PU_(W) (1≤z≤W) of the abovementioned “prediction unit” type. Such asubdivision can be conventional or else in accordance with theinvention, as represented in FIGS. 5A and 5B. In a way similar to whatwas described with reference to the embodiment of FIG. 6A, a successionof indicators representative of the subdivision is intended to betransmitted in the data signal F.

During an optional substep C625 represented in FIG. 6B, the codingmodule UCO of FIG. 4 selects a first current subpart PU_(z). Such aselection is made in a predefined order, such as, for example,lexicographic order.

During an optional substep C626 represented in FIG. 6B, the PRED_COmodule of FIG. 4 selects, for the current subpart PU_(z) the optimalprediction parameters associated with the prediction mode selected inthe abovementioned substep C623. If, for example, the inter-predictionmode was selected in the abovementioned substep C623, the optimalprediction parameters are one or more motion vectors, as well as one ormore reference images, such optimal parameters making it possible toobtain the best performance levels in coding of the current subpartPU_(z) according to a predetermined criterion, such as, for example, therate-distortion criterion. If, for example, the intra-prediction modewas selected in the abovementioned substep C623, the optimal predictionparameters are associated with an intra mode selected from differentavailable intra modes. As for the inter mode, the optimal predictionparameters are those which make it possible to obtain the bestperformance levels in coding of the current subpart PU_(z) according toa predetermined criterion, such as, for example, the rate-distortioncriterion.

The substeps C625 to C626 are iterated for each of the subparts PU₁,PU₂, PU_(z), . . . , PU_(W) of the current subpart CU_(k′) of the firstpart CU₁ of the current block CTU_(u), in the predeterminedlexicographic order.

During an optional substep C627 represented in FIG. 6B, the partitioningmodule MP_CO of FIG. 4 subdivides the current subpart CU_(k′) into aplurality Z of transform subparts TU₁, TU₂, . . . , TU_(w), . . . TU_(Z)(1≤w≤Z) of the abovementioned “transform unit” type. Such a subdivisioncan be conventional or else in accordance with the invention, asrepresented in FIGS. 5A and 5B. In a way similar to what was describedwith reference to the embodiment of FIG. 6A, a succession of indicatorsrepresentative of the subdivision is intended to be transmitted in thedata signal F.

During an optional substep C628 represented in FIG. 6B, the codingmodule UCO of FIG. 4 selects a first current transform subpart TU_(w).Such a selection is performed in a predefined order, such as, forexample, lexicographic order.

During a substep C629 represented in FIG. 6B, the computation moduleCAL3_CO of FIG. 4 proceeds, in a way similar to the substep C612 of FIG.6A, with the computation of a residual subpart TUr_(w).

During a substep C630 represented in FIG. 6B, the MT_CO module of FIG. 4proceeds with the transformation of the residual subpart TUr_(w)according to a conventional direct transformation operation, such as,for example, a discrete cosine transformation of DCT type, to produce atransform subpart TUt_(w).

During a substep C631 represented in FIG. 6B, the MQ_CO module of FIG. 4proceeds with the quantization of the transform subpart TUt_(w)according to a conventional quantization operation, such as, forexample, a scalar quantization. A subpart TUq_(w), formed by quantizedcoefficients, is then obtained.

During a substep C632 represented in FIG. 6B, the MCE_CO module of FIG.4 proceeds with the entropic coding of the quantized coefficientsTUq_(w).

The set of substeps C628 to C632 is iterated for each of the subpartsTU₁, TU₂, . . . , TU_(w), . . . , TU_(Z) of the current subpart CU_(k′)of the first part CU₁ of the current block CTU_(u), in the predeterminedlexicographic order.

According to the invention, in the case where the current transformsubpart TU_(w) has a geometrical form with m sides, an intermediatesubstep C6290 is implemented between the abovementioned substeps C629and C630. During this intermediate substep, the residual pixels of theresidue sub-part TUr_(w) with m sides are complemented with pixels ofzero value or coded according to a predetermined coding method, until asquare or rectangular block of pixels is obtained.

The abovementioned substep C6290 is implemented by the computationsoftware module CAL4_CO as represented in FIG. 4.

If the computation substep C6290 is implemented during the substep C631represented in FIG. 6B, the MQ_CO module of FIG. 4 proceeds with thequantization of the current transform subpart TUt_(w) to the exclusionof the pixels added during the substep C6290 and which have undergone atransformation during the substep C630.

The set of substeps C622 to C632 is iterated for each of the subpartsCU₁, CU₂, . . . CU_(k′), . . . , CU_(N) of the current first part CU₁ ofthe current block CTU_(u), in the predetermined lexicographic order.

Detailed Description of the Decoding Part

An embodiment of the invention will now be described, in which thedecoding method according to the invention is used to decode a datasignal representative of an image or of a sequence of images which iscapable of being decoded by a decoder conforming to any one of thecurrent or future video decoding standards.

In this embodiment, the decoding method according to the invention isfor example implemented by software or hardware by modifications of sucha decoder. The decoding method according to the invention is representedin the form of an algorithm comprising steps D1 to D7 as represented inFIG. 8.

According to the embodiment of the invention, the decoding methodaccording to the invention is implemented in a decoding device ordecoder DO represented in FIG. 9. s illustrated in FIG. 9, according tothis embodiment of the invention, the decoder DO comprises a memoryMEM_DO which itself comprises a buffer memory TAMP_DO, a processing unitUT_DO equipped for example with a microprocessor μP and driven by acomputer program PG_DO which implements the decoding method according tothe invention. On initialization, the code instructions of the computerprogram PG_DO are for example loaded into a RAM memory before beingexecuted by the processor of the processing unit UT_DO.

The decoding method represented in FIG. 8 is applied to a data signalrepresentative of a fixed current image IC_(j) to be decoded or of asequence of images to be decoded.

To this end, information representative of the current image IC_(j) tobe decoded is identified in the data signal F received on the decoderDO, as delivered following the coding method of FIG. 3.

Referring to FIG. 8, during a step D1, there are identified in thesignal F, quantized blocks CTUq₁, CTUq₂, . . . , CTUq_(u), . . . ,CTUq_(S) (1≤u≤S) associated respectively with the blocks CTU₁, CTU₂, . .. , CTU_(u), . . . , CTU_(S) coded previously according to theabovementioned lexicographic order, according to the coding method ofFIG. 3.

Such an identification step is implemented by a flow analysisidentification processor or software module MI_DO, as represented inFIG. 9, said module being driven by the microprocessor μP of theprocessing unit UT_DO.

Other types of scanning than that which has just been described aboveare of course possible and depend on the scan order chosen in decoding.

Preferentially, each of the blocks to be decoded CTU₁, CTU₂, . . . ,CTU_(u), . . . , CTU_(S) has a square form and comprises N×N pixels,where N≤2.

According to an alternative, each of the blocks to be decoded CTU₁,CTU₂, . . . , CTU_(u), . . . , CTU_(S) has a rectangular form andcomprises N×P pixels, where N≥1 and P≥2.

During a step D2 represented in FIG. 8, the decoder DO of FIG. 9 selectsas current block the first quantized block CTUq_(u) which containsquantized data which have been coded during the step C6 of FIG. 3.

During a step D3 represented in FIG. 8, together with the quantizedblock CTUq_(u) which has been selected, the compressed value of thesyntax element type_decoupe which was selected on completion of the stepC4 of FIG. 3 is read, together, if necessary, with the compressed valueof the syntax element arr_decoupe1 or arr_decoupe2 which is associatedwith it.

As explained above in the description, the syntax element type_decoupedesignates the indicator representative of a given subdivision mode.According to a preferential embodiment, the syntax element type_decoupetakes for example three values:

-   -   0 to indicate a conventional subdivision of the current block        into four rectangular or square blocks,    -   1 to indicate a subdivision of the current block in accordance        with the subdivision mode represented in FIG. 5A,    -   2 to indicate a subdivision of the current block in accordance        with the subdivision mode represented in FIG. 5B,    -   3 to indicate an absence of subdivision of the current block.

The reading step D3 is performed by a reading processor or softwaremodule ML_DO, such as represented in FIG. 9, which module is driven bythe microprocessor μP of the processing unit UT_DO.

In a way identical to the coder CO of FIG. 4, the buffer memory TAMP_DOof the coder DO of FIG. 9 has stored in it:

-   -   a predefined list LT_(a) of several sets of coordinates each        defining a rectangular block of a predetermined form,    -   a predefined list LT_(b) of several sets of coordinates each        defining a rectangular block of a predetermined form,    -   a look-up table TC.

During a step D4 represented in FIG. 8, the value of the syntax elementtype_decoupe which was read in the abovementioned step D3 is decodedtogether, if necessary, with the decoding of the value of the syntaxelement arr_decoupe1 or arr_decoupe2 which is associated with it.

The abovementioned step D4 is implemented by an indicator decodingprocessor or software module MDI as represented in FIG. 9, which moduleis driven by the microprocessor μP of the processing unit UT_DO.

During a step D5 represented in FIG. 8, the current block CTU issubdivided into at least one first part CU₁ and one second part CU₂, thefirst and second parts complementing one another. According to theinvention:

-   -   the first part CU₁ has a rectangular or square form,    -   and the second part CU₂ has a geometrical form with m sides,        where m>4.

According to a preferred embodiment, the current block CTU issubdivided:

-   -   into a first part CU₁ of rectangular or square form or else into        a plurality of parts of rectangular or square form,    -   and into at most one second part CU₂ of a geometrical form with        m sides.

Examples of subdivision have been presented with reference to FIGS. 5Aand 5B above and will not be described again here.

The subdivision step D5 is performed by a partitioning processor orsoftware module MP_DO, as represented in FIG. 9, which module is drivenby the microprocessor μP of the processing unit UT_DO.

During a step D6 represented in FIG. 8, the parts CU₁ and CU₂ of thecurrent block CTU_(u) to be decoded are decoded according to apredetermined scan order. According to a preferred embodiment, the firstpart CU₁ is decoded before the second part CU₂. Alternatively, the firstpart CU₁ is decoded after the second part CU₂.

The decoding step D6 is implemented by a decoding processor or softwaremodule UDO as represented in FIG. 9, which module is driven by themicroprocessor μP of the processing unit UT_DO.

As represented in more detail in FIG. 9, the decoding module UDOconventionally comprises:

-   -   an entropic decoding processor or software module MDE_DO, for        example of CABAC (“context adaptive binary arithmetic coder”)        type, or even a Huffman decoder known as such,    -   a dequantization processor or software module MQ1 ⁻¹_DO,    -   an inverse transformation processor or software module MT1 ⁻¹_DO        of DCT⁻¹ (discrete cosine transform), DST⁻¹ (discrete sine        transform), DWT⁻¹ (discrete wavelet transform) type,    -   an inverse prediction processor or software module PRED1 ⁻¹_DO,    -   a block reconstruction computation processor or module CAL2_DO.

On completion of the step D6, a current decoded block CTUD_(u) isobtained.

During a step D7 represented in FIG. 8, said decoded block CTUD_(u) iswritten into a decoded image ID_(j).

Such a step is implemented by an image reconstruction processor orsoftware module URI as represented in FIG. 9, said module being drivenby the microprocessor μP of the processing module UT_DO.

The decoding steps which have just been described above are implementedfor all the blocks CTU₁, CTU₂, . . . , CTU_(u), . . . , CTU_(S) to bedecoded of the current image IC_(j) considered, in a predetermined orderwhich is, for example, the lexicographic order.

Other run-through types than that which has just been described aboveare of course possible.

There now follows a description, with reference to FIG. 10A, of a firstembodiment of the different substeps implemented during theabovementioned decoding step D6, in the decoding module UDO representedin FIG. 9.

According to this first embodiment, the data signal F contains thepartitioning indicators of a current block CTU_(u) which has been codedaccording to the embodiment of FIG. 6A. To this end, as described abovein the description associated with the embodiment of FIG. 6A, the signalF contains the following four values 1133 which have been decoded oncompletion of the abovementioned step D4 and which are representative:

-   -   of the partitioning of the current block CTU_(u) according to        one of the subdivision modes represented in FIG. 5A and more        specifically according to the type of subdivision SUBD2 ₁ of        FIG. 5A,    -   and of the absence of subdivision of the parts CU₁ and CU₂ of        the current block CTU_(u).

As a variant, the data signal F contains the following three values 113,in the case where the indicator type_decoupe of value 3 associated withthe coded data of the second part CU₂ has not been entered into the datasignal F, given the fact that the second part CU₂ defines a uniform zoneof the current block CTU_(u).

Consequently, the indicator type_decoupe is systematically set to thepredetermined value 3, such that the second part CU₂ is not subdividedin the decoding.

During a substep D610 represented in FIG. 10A, the decoding module UDOselects, as current set of quantized coefficients CUq_(k) associatedwith the current part CU_(k) (k=1 or k=2), either the set of quantizedcoefficients associated with the square part CU₁ first, or the set ofquantized coefficients associated with the part CU₂ with m sides first.

During a substep D611 represented in FIG. 10A, there is an entropicdecoding of the current set of quantized coefficients CUq₁ associatedwith the first part CU₁. In the preferred embodiment, the decodingperformed is an entropic decoding of arithmetic or Huffman type. Thesubstep D611 then consists in:

-   -   reading the symbol or symbols of a predetermined set of symbols        which are associated with the current set of quantized        coefficients Cuq₁,    -   associating numeric information, such as bits, with the        symbol(s) read.

On completion of the abovementioned substep D611, a plurality of numericinformation associated with the current set of quantized coefficientsCuq₁ is obtained.

Such an entropic decoding substep D611 is implemented by the entropicdecoding module MDE_DO represented in FIG. 9.

During the abovementioned substep D611, there is also the decoding ofthe information relating to the predictive coding of the part CU₁ asimplemented in the substep C611 of FIG. 6A, and which was entered intothe data signal F. Such reconstruction information notably comprises theprediction type (inter- or intra-prediction), and if appropriate, theintra-prediction mode or else the reference image index and the motionvector used in the inter-prediction mode.

During a substep D612 represented in FIG. 10A, the numeric informationobtained following the substep D611 is dequantized, according to aconventional dequantization operation which is the reverse operation ofthe quantization implemented during the quantization substep C614 ofFIG. 6A. A current set of dequantized coefficients CUDq₁ is thenobtained on completion of the substep D612. Such a substep D612 isperformed by means of the dequantization module MQ⁻¹_DO, as representedin FIG. 9.

During a substep D613 represented in FIG. 10A, the current set ofdequantized coefficients CUDq₁ is transformed, such a transformationbeing a direct inverse transformation, such as, for example, an inversediscrete cosine transformation of DCT⁻¹ type. This transformation is thereverse operation of the transformation performed in the substep C613 ofFIG. 6A. On completion of the substep D613, a decoded residual partCUDr₁ is obtained. Such an operation is performed by the module MT⁻¹_DOrepresented in FIG. 9.

During a substep D614 represented in FIG. 10A, the PRED⁻¹_DO module ofFIG. 9 proceeds with the predictive decoding of the current part CU₁using information relating to the predictive coding of the part CU₁which was decoded during the abovementioned substep D611.

Said abovementioned substep of predictive decoding makes it possible toconstruct a predicted part CUDp₁ which is an approximation of thecurrent part CU₁ to be decoded.

During a substep D615 represented in FIG. 10A, the CAL2_DO module ofFIG. 9 proceeds with the reconstruction of the current part CU₁ byadding to the decoded residual part CUDr₁, obtained on completion of thesubstep D613, the predicted part CUDp₁ which was obtained on completionof the abovementioned substep D614.

The abovementioned substeps D610 to D615 are then iterated with a viewto decoding the second part CU₂ with m sides of the current blockCTU_(u).

In accordance with the invention, in the case of the decoding of thesecond part CU₂ with m sides, one or more items of information onreconstruction of the pixels of the second part CU₂ are set topredetermined values.

Thus, preferentially, during the substep D614 of predictive decoding ofthe part CU₂ of the current block CTU_(u), the pixels of the part CU₂ tobe decoded are predicted relative respectively to pixels ofpredetermined corresponding values. Such values are stored in a list LPcontained in the buffer memory TAMP_DO of the decoder DO of FIG. 9.

According to a preferred variant embodiment, the substep D610 of FIG.10A is not implemented since no set of quantized coefficients associatedwith the part CU₂ with m sides has been transmitted in the data signalF. The quantized coefficients of the quantized residual part CUq₂ arethen directly all set to zero by the decoding module UDO of FIG. 9.

Such an arrangement is made advantageous by the fact that the part CU₂of the current block CTU_(u) which has been coded is considered uniform.

According to another preferred variant embodiment, the abovementionedsubstep D611 is not completely implemented, the decoder DO directlydeducing, following the abovementioned substep D610, predeterminedvalues of reconstruction information associated with the residual partCUr₂.

Such an arrangement is made advantageous by the fact that the part CU₂of the current block CTU_(u) which has been coded is considered uniform.

As a variant, the pixels of the part CU₂ to be decoded are predictedconventionally, in the same way as the part CU₁.

In accordance with the invention, between the abovementioned substepsD611 and D612, an intermediate step D6110 is implemented. During thisintermediate step, the decoded pixel values which have been obtainedfollowing the step of entropic decoding of the plurality of numericinformation associated with the current set of quantized coefficientsCUq₂ are complemented with predetermined pixel values, until a square orrectangular block of pixel values is obtained.

According to different possible embodiments, the pixel values associatedwith the current set of quantized coefficients CUq₂ can be complemented:

-   -   with respective zero pixel values,    -   with pixel values reconstructed conventionally by interpolation,    -   with pixel values reconstructed conventionally using the        so-called “inpaiting” technique.

The abovementioned substep D6110 is implemented by a computationsoftware module CAL1_DO as represented in FIG. 9, which module is drivenby the microprocessor μP of the processing unit UT_DO.

Such an arrangement makes it possible to re-use the transformationsoftware module MT⁻¹_DO of FIG. 9 which conventionally applies square orrectangular block transforms.

Given the fact that the substep D6110 is applied only for the decodedpixel values which have been obtained following the step of entropicdecoding of the plurality of numeric information associated with thecurrent set of quantized coefficients CUq₂ of a geometrical form with msides, this step, like the computation module CAL1_DO, are representedby dotted lines, respectively in FIGS. 10A and 9.

There now follows a description, referring to FIG. 10B, of a secondembodiment of the different substeps implemented during theabovementioned decoding step D6, in the coding module UDO represented inFIG. 9.

This second embodiment is distinguished from that of FIG. 10A by thefact that the first part CU₁ to be decoded of the current block CTU_(u)is subdivided again.

According to this second embodiment, the data signal F contains thepartitioning indicators of a current block CTU_(u)which has been codedaccording to the embodiment of FIG. 6B. To this end, as described abovein the description associated with the embodiment of FIG. 6B, the signalF contains the following fifteen values 113032603333333 as representedin FIG. 7 and which have been decoded on completion of theabovementioned step D4.

Such values are representative:

-   -   of the partitioning of the current block CTU_(u) according to        one of the subdivision modes represented in FIG. 5A and more        specifically according to the type of subdivision SUBD2 ₁ of        FIG. 5A,    -   of the absence of subdivision of the second part CU₂ with m        sides of the current block CTU_(u),    -   of the subdivision of the first part CU₁ of the current block        CTU_(u) as represented in FIG. 7.

As a variant, the data signal F does not contain the two values equal to3 represented in bold, in the case where:

-   -   the indicator type_decoupe of value 3 associated with the coded        data of the second part CU₂ has not been entered into the data        signal F, given the fact that the second part CU₂ defines a        uniform zone of the current block CTU_(u),    -   the indicator type_decoupe of value 3 associated with the coded        data of the second part CU22 ₁ with m sides of the block CU2 ₁        as represented in FIG. 7, given the fact that the second part        CU22 ₁ defines a uniform zone of the block CU2 ₁.

Consequently, the indicator type_decoupe is systematically set to thepredetermined value 3, such that neither the second part CU₂ of thecurrent block CTU_(u) to be decoded, nor the second part CU22 ₁ with msides of the block CU2 ₁ of the current block CTU_(u) to be decoded, issubdivided in the decoding.

During a substep D620 represented in FIG. 10B, the decoding module UDOselects as current set of quantized coefficients CUq_(k) associated withthe current part CU_(k) (k=1 or k=2), either the set of quantizedcoefficients associated with the square part CU₁ first, or the set ofquantized coefficients associated with the part CU₂ with m sides first.

During a substep D621 represented in FIG. 10B, the decoding module UDOtests whether the index k associated with the current part CU_(k) to bedecoded has the value 1 or 2.

If the index k is equal to 2, the part CU₂ of the current block CTU tobe decoded is decoded according to the substeps D610 to D615 of FIG.10A.

If the index k is equal to 1, during a substep D622 represented in FIG.10B, the decoding module UDO of FIG. 9 selects a current subpart CU_(k′)to be decoded of the first part CU₁ of the current block CTU_(u) to bedecoded, such that 1≥k′≥N.

In the example represented in FIG. 7, N=8, since the first part CU₁ ofthe current block CTU_(u) has been subdivided into eight subparts of“coding unit” type CU1 ₁, CU21 ₁, CU2 ₁, CU31 ₁, CU32 ₁, CU33 ₁, CU34₁,CU4 ₁.

During a substep D623 represented in FIG. 10B, the entropic decodingmodule MDE_DO of FIG. 9 proceeds with an entropic decoding of thecurrent set of quantized coefficients CUq_(w) associated with thecurrent subpart CU_(k′) of the first part CU₁ of the current blockCTU_(u) to be decoded. In the preferred embodiment, the decodingperformed is an entropic decoding of arithmetic or Huffman type. Thesubstep D623 then consists in:

-   -   reading the symbol or symbols of a predetermined set of symbols        which are associated with the current set of quantized        coefficients CUq_(w),    -   associating numeric information, such as bits, with the        symbol(s) read.

On completion of the abovementioned substep D623, a plurality of numericitems of information associated with the current set of quantizedcoefficients CUq_(k′) is obtained.

During the substep D623, the entropic decoding module MDE_DO of FIG. 9proceeds also with an entropic decoding of the indicator IPRrepresentative of the inter- or intra-prediction mode which has beenselected for this current subpart CU_(k′) during the substep C623 ofFIG. 6B.

During an optional substep D624 represented in FIG. 10B, in the casewhere the current subpart CU_(k′) to be decoded has been subdividedduring the substep C626 of FIG. 6B into a plurality W of predictionsubparts PU₁, PU₂, . . . , PU_(z), . . . PU_(W) (1≤z≤W), the readingsoftware module ML_DO of FIG. 9 proceeds to read the compressed value ofthe indicator representative of such a subdivision. Such an indicatorconsists of the syntax element type_decoupe and, if appropriate, of thesyntax element arr_decoupe1 or arr_decoupe2 which is associated with it.

During an optional substep D625 represented in FIG. 10B, the indicatordecoding software module MDI of FIG. 9 proceeds with the decoding of thevalue of the syntax element type_decoupe which was read in theabovementioned substep D624 and, if appropriate, with the decoding ofthe value of the syntax element arr_decoupe1 or arr_decoupe2 which isassociated with it.

During an optional substep D626 represented in FIG. 10B, thepartitioning software module MP_DO of FIG. 9 subdivides the currentsubpart CU_(k′) to be decoded into a plurality W of prediction subpartsPU₁, PU₂, . . . , PU_(z), . . . , PU_(W) (1≤z≤W).

During an optional substep D627 represented in FIG. 10B, the decodingmodule UDO of FIG. 9 selects a first current subpart PU_(z). Such aselection is performed in a predefined order, such as, for example, thelexicographic order.

During an optional substep D628 represented in FIG. 6B, the entropicdecoding module MDE_DO of FIG. 9 proceeds, in association with thecurrent subpart PU_(z), with an entropic decoding of the optimalprediction parameters which were selected during the substep C626 ofFIG. 6B, in association with the indicator I_(PR) which isrepresentative of the prediction mode selected in the abovementionedsubstep C623 and which was decoded in the substep D623. If, for example,the INTER-prediction mode was selected in the abovementioned substepC623, the decoded optimal prediction parameters are one or more motionvectors, and one or more reference images. If, for example, theINTRA-prediction mode was selected in the abovementioned substep C623,the optimal prediction parameters are associated with an INTRA modeselected from different available INTRA modes.

The substeps D627 to D628 are iterated for each of the subparts PU₁,PU₂, . . . , PU_(z), . . ., PU_(W) of the current subpart CU_(k′) to bedecoded of the first part CU₁ of the current block CTU_(u), in thepredetermined lexicographic order.

During an optional substep D629 represented in FIG. 10B, in the casewhere the current subpart CU_(k′) to be decoded has been subdivided,during the substep C627 of FIG. 6B, into a plurality Z of transformsubparts TU₁, TU₂, . . . , TU_(w), . . . TU_(Z) (1≤w≤Z), the readingsoftware module ML_DO of FIG. 9 proceeds to read the compressed value ofthe indicator representative of such a subdivision. Such an indicatorconsists of the syntax element type_decoupe and, if appropriate, of thesyntax element arr_decoupe1 or arr_decoupe2 which is associated with it.

During an optional substep D630 represented in FIG. 10B, the indicatordecoding software module MDI of FIG. 9 proceeds with the decoding of thevalue of the syntax element type_decoupe which was read in theabovementioned substep D629 and, if appropriate, with the decoding ofthe value of the syntax element arr_decoupe1 or arr_decoupe2 which isassociated with it.

During an optional substep D631 represented in FIG. 10B, thepartitioning software module MP_DO of FIG. 9 subdivides the currentsubpart CU_(k′) to be decoded into a plurality Z of transform subpartsTU₁, TU₂, . . . , TU_(w), . . . , TU_(Z) (1≤w≤Z).

During an optional substep D632 represented in FIG. 6B, the decodingmodule UDO of FIG. 9 selects the current set of quantized coefficientsTUq_(w) associated with the first current transform subpart TU_(w). Sucha selection is performed in a predefined order, such as, for example,the lexicographic order.

During a substep D633 represented in FIG. 6B, the entropic decodingmodule MDE_DO of FIG. 9 proceeds with an entropic decoding of thecurrent set of quantized coefficients TUq_(w) associated with the firstcurrent transform subpart TU_(w) to be decoded. In the preferredembodiment, the decoding performed is an entropic decoding of arithmeticor Huffman type. The substep D633 then consists in:

-   -   reading the symbol or symbols of a predetermined set of symbols        which are associated with the current set of quantized        coefficients Cuq₁,    -   associating numeric information, such as bits, with the        symbol(s) read.

On completion of the abovementioned substep D633, a plurality of numericitems of information associated with the current set of quantizedcoefficients TUq_(w) is obtained.

During a substep D634 represented in FIG. 10B, the dequantization moduleMQ⁻¹_DO of FIG. 9 proceeds with the dequantization of the numericinformation obtained following the substep D633, according to aconventional dequantization operation which is the reverse operation ofthe quantization implemented during the quantization substep C631 ofFIG. 6B. A current set of dequantized coefficients TUDq_(w) is thenobtained on completion of the substep D634.

During a substep D635 represented in FIG. 10B, the module MT⁻¹_DO ofFIG. 9 proceeds with a transformation of the current set of dequantizedcoefficients TUDq_(w), such a transformation being an inverse directtransformation, such as, for example, an inverse discrete cosinetransformation of DCT⁻¹ type. This transformation is the reverseoperation of the transformation performed in the substep C630 of FIG.6A. On completion of the substep D635, a decoded residual part TUDr_(w)is obtained.

During a substep D636 represented in FIG. 10B, the PRED⁻¹_DO module ofFIG. 9 proceeds with the predictive decoding of the first currenttransform subpart TU_(w) using optimal prediction parameters which wereread during the abovementioned substep D628.

Said abovementioned predictive decoding substep makes it possible toconstruct a first current predicted transform subpart TUDp_(w) which isan approximation of the first current transform subpart TU_(w) to bedecoded.

During a substep D637 represented in FIG. 10B, the CAL2_DO module ofFIG. 9 proceeds with the reconstruction of the first current transformsubpart TU_(w) by adding to the decoded residual part TUDr_(w), obtainedon completion of the substep D635, the predicted part TUDp_(w) which wasobtained on completion of the abovementioned substep D636.

The set of substeps D632 to D637 is iterated for each of the subpartsTU1, TU2, . . . , TU_(w), . . . , TU_(Z) to be decoded of the currentsubpart CU_(k′) to be decoded of the first part CU₁ of the current blockCTU_(u), in the predetermined lexicographic order.

According to the invention, in the case where the current transformsubpart TU_(w) has a geometrical form with m sides, an intermediatesubstep D6330 is implemented between the abovementioned substeps D633and D634. During this intermediate substep, the decoded pixel valueswhich were obtained following the substep D633 of entropic decoding ofthe plurality of numeric items of information associated with thecurrent set of quantized coefficients TUq_(w) are complemented withpredetermined pixel values, until a square or rectangular block of pixelvalues is obtained.

The abovementioned substep D6330 is implemented by the computationsoftware module CAL1_DO as represented in FIG. 9.

The set of the substeps D622 to D637 is iterated for each of thesubparts CU₁, CU₂, . . . , CU_(k′), . . . , CU_(N) to be decoded of thefirst current part CU₁ of the current block CTU_(u), in thepredetermined lexicographic order.

An exemplary embodiment of the invention remedies drawbacks of theabovementioned prior art.

It goes without saying that the embodiments which have been describedabove have been given in a purely indicative and nonlimiting manner, andthat numerous modifications can easily be made by a person skilled inthe art without in any way departing from the scope of the invention.

What is claimed is:
 1. A method comprising: coding at least one image bya coding device, comprising: subdividing the at least one image into aplurality of blocks; subdividing at least one current block into a firstpart and a second part according to a given subdivision mode for said atleast one current block, for which the first part has a rectangular orsquare form and the second part forms the complement of the first partin the current block, said second part having a geometrical form with msides, where m>4, said second part defining a uniform zone of said atleast one current block; coding a value of a syntax element indicatingsaid given subdivision mode; coding the first part, obtaining coded dataassociated to said first part, said coded data being representative of asignificant element of said at least one current block; coding thesecond part with m sides, obtaining coded data associated to said secondpart with m sides, in which at least one item of information ofreconstruction of said second part with m sides is set to apredetermined zero value of a prediction residue of said second partwith m sides; and transmitting to a decoder a data signal including thecoded value of said syntax element, said value of said syntax elementbeing representative of said subdivision of said at least one currentblock into said first part and said second part, said coded dataassociated to said first part, and said coded data associated to saidsecond part; the predetermined zero value of said prediction residue ofsaid second part with m sides being not contained in said data signal.2. A coding device comprising: a non-transitory computer-readable mediumcomprising instructions stored thereon; and a processor configured bythe instructions to code at least one image by performing actscomprising: subdividing the image into a plurality of blocks;subdividing at least one current block into a first part and a secondpart, according to a given subdivision mode for said at least onecurrent block, for which the first part has a rectangular or square formand the second part forms the complement of the first part in thecurrent block, said second part having a geometrical form with m sides,where m>4, said second part defining a uniform zone of said at least onecurrent block; coding a value of a syntax element indicating said givensubdivision mode; coding the first part, obtaining coded data associatedto said first part, said coded data being representative of asignificant element of said at least one current block; coding thesecond part with m sides, obtaining coded data associated to said secondpart with m sides, in which least one item of information ofreconstruction of said second part with m sides is set to apredetermined zero value of a prediction residue of said second partwith m sides; and transmitting to a decoder a data signal including thecoded value of said syntax element, said value of said syntax elementbeing representative of said subdivision of said at least one currentblock into said first part and said second part, said coded dataassociated to said first part, and said coded data associated to saidsecond part; said predetermined zero value of said prediction residue ofsaid second part with m sides being not contained in said data signal.3. A non-transitory computer-readable medium comprising a computerprogram stored thereon and comprising instructions for coding at leastone image when the instructions are run on a computer of a codingdevice, wherein the instructions configure the coding device to performacts comprising: subdividing the at least one image into a plurality ofblocks; subdividing at least one current block into a first part and asecond part, according to a given subdivision mode for said at least onecurrent block, for which the first part has a rectangular or square formand the second part forms the complement of the first part in thecurrent block, said second part having a geometrical form with m sides,where m>4, said second part defining a uniform zone of said at least onecurrent block; coding a value of a syntax element indicating said givensubdivision mode; coding the first part, obtaining coded data associatedto said first part, said coded data being representative of asignificant element of said at least one current block; coding thesecond part with m sides, obtaining coded data associated to said secondpart with m sides, in which at least one item of information ofreconstruction of said second part with m sides is set to apredetermined zero value of a prediction residue of said second partwith m sides; transmitting to a decoder a data signal including thecoded value of said syntax element, said value of said syntax elementbeing representative of said subdivision of said at least one currentblock into said first part and said second part, said coded dataassociated to said first part, and said coded data associated to saidsecond part with m sides; and said predetermined zero value of said aprediction residue of said second part with m sides being not containedin said data signal.
 4. A decoding method comprising: decoding a datasignal representative of at least one coded image having been subdividedinto a plurality of blocks, wherein decoding comprises the followingacts performed by a decoding device, for at least one current block tobe decoded: reading, in said data signal, a syntax element indicating asubdivision mode of said at least one current block; decoding a value ofsaid syntax element, said value of said syntax element beingrepresentative of a subdivision of said at least one current block intoa first part and a second part, the first part having a rectangular orsquare form and the second part forming the complement of the first partin the current block, said second part having a geometrical form with msides, where m>4; subdividing said at least one current block accordingsaid decoded value of said syntax element, selecting, in said datasignal, coded data of said first part; decoding said selected coded dataof said first part, said decoded data being representative of asignificant element of said at least one current block; and decodingcoded data of said second part with m sides, said second part with msides defining a uniform zone of said at least one current block, inwhich the following acts are performed: setting at least one item ofinformation of reconstruction of said second part with m sides to apredetermined zero value of a prediction residue of said second partwith m sides, said predetermined zero value of said prediction residuebeing not contained in said data signal; and reconstructing said secondpart with m sides according to said predetermined zero value.
 5. Thedecoding method as claimed in claim 4, in which, at least one item ofinformation of reconstruction of said second part with m sidescorresponds to some values of prediction pixels which are stored in thedecoding device.
 6. The decoding method as claimed in claim 4, in whichsaid at least one item of information of reconstruction of the secondpart with m sides of the current block is representative of an act ofnot subdividing said second part with m sides of the current block. 7.The decoding method as claimed in claim 4, in which said act of readingcomprises reading, in the data signal, an item of information indicatingwhether the current block is intended to be subdivided into said firstand second parts or else according to another predetermined method. 8.The decoding method as claimed in claim 4, in which said act of readingcomprises reading, in the data signal, an item of information indicatinga given subdivision configuration of the current block into said firstand second parts, said configuration being selected from variouspredetermined subdivision configurations.
 9. The decoding method asclaimed in claim 4, in which the act of decoding of coded data of thesecond part with m sides of the current block comprises in the followingsub-acts: applying an entropic decoding to the pixels of said secondpart with m sides; and complementing the entropically decoded pixels ofsaid second part with m sides with pixels reconstructed according to apredetermined reconstruction method, until a square or rectangular blockof pixels is obtained.
 10. The decoding method as claimed in claim 4, inwhich a subdivided current block contains at most a part having ageometrical form with m sides.
 11. A decoding device comprising: anon-transitory computer-readable medium comprising instructions storedthereon; and a processor configured by the instructions to decode a datasignal representative of at least one coded image having been subdividedinto a plurality of blocks, wherein decoding comprises, for at least onecurrent block to be decoded: reading, in said data signal, a syntaxelement indicating a subdivision mode of said at least one currentblock; decoding a value of said syntax element, said value of saidsyntax element being representative of a subdivision of said at leastone current block into a first part and a second part, the first parthaving a rectangular or square form and the second part forming thecomplement of the first part in the current block, said second parthaving a geometrical form with m sides, where m>4; subdividing said atleast one current block according said decoded value of said syntaxelement, selecting, in said data signal, coded data of said first part;decoding said selected coded data of said first part, said decoded databeing representative of a significant element of said at least onecurrent block; and decoding coded data of said second part with m sides,said second part with m sides defining a uniform zone of said at leastone current block, in which the following acts are performed: setting atleast one item of information of reconstruction of said second part withm sides to a predetermined zero value of a prediction residue of saidsecond part with m sides, said predetermined zero value of saidprediction residue being not contained in said data signal; andreconstructing said second part with m sides according to saidpredetermined zero value.
 12. A non-transitory computer-readable mediumcomprising a computer program stored thereon and comprising instructionsfor decoding a data signal representative of at least one coded imagehaving been subdivided into a plurality of blocks, when the instructionsare run on a computer of a decoding device, wherein the instructionsconfigure the decoding device to perform, for at least one current blockto be decoded, the acts comprising: reading, in said data signal, asyntax element indicating a subdivision mode of said at least onecurrent block; decoding a value of said syntax element, said value ofsaid syntax element being representative of a subdivision of said atleast one current block into a first part and a second part, the firstpart having a rectangular or square form and the second part forming thecomplement of the first part in the current block, said second parthaving a geometrical form with m sides, where m>4; subdividing said atleast one current block according said decoded value of said syntaxelement, selecting, in said data signal, coded data of said first part;decoding said selected coded data of said first part, said decoded databeing representative of a significant element of said at least onecurrent block; and decoding coded data of said second part with m sides,said second part with m sides defining a uniform zone of said at leastone current block, in which the following acts are performed: setting atleast one item of information of reconstruction of said second part withm sides to a predetermined zero value of a prediction residue of saidsecond part with m sides, said predetermined zero value of saidprediction residue being not contained in said data signal; andreconstructing said second part with m sides according to saidpredetermined zero value.